This web site is configured so that only users with administrator privileges can create user accounts.
This policy wasn't set to be annoying or maintain exclusivity. Web applications that allow anonymous users to create accounts tend to quickly get overrun with spam. A few years ago, in unrelated attacks, the Spartan Robotics forums and the Los Altos Robotics website were both filled with bogus accounts by bots. These bogus accounts were used to post the kinds of not-exactly-clean content normally found in your spam folder.
Please don't reconfigure the site to allow anonymous user to create accounts. The attacks that hurt Los Altos Robotics and the Spartan Robotics forum system are incredibly common and very time-consuming to clean up after-the-fact. If seriously objectionable content is inserted by a bogus account, the team's reputation could be significantly damaged.
If you're logged in as an administrator, there will be a "People" section accessible from the top navigation bar. Under the "People" section, there is an "Add user" link that takes you to a page that lets you manually create a user account for someone. To change a password, go to the "People" page and click on edit to the right of the username you want to update.
Note: If you want to let the user create edit the site content, you currently have to check the box for adding the "Adminstrator" to their account. Beware that the "Administrator" role gives them full control over the site. If you want a user to be able to edit content, but not have full control over the site, you'll have to set up a custom role.